There are numerous dies for and methods of extruding plastic film in current use which employ electrical resistance type heaters fastened against the outer surface of the die. Although such resistance heaters are very simple and economical they do not always provide uniform heating of the die being heated.
The main reasons for this lack of uniform heating are the nonuniformity of heat evolution from the resistance heater itself which is inherent in any such heater and the usually spotty contact between the resistance heater and the surface it engages. The nonuniformity of heat evolution in the resistance heater is due to normal variations in the gauge of the electrical resistance wire, wire spacing, insulation between wires or heating elements and in the case of a band heater the usual gap at the end of the band. The spotty contact is due to the fact that, in general, the heater and heated surfaces are both usually made of stiff metallic materials whereby adjoining surfaces do not readily conform to each other to provide optimum contact therebetween. Accordingly, it has been found that conductive heat transfer to the areas of contact is good while the locations not being contacted by the heater must be heated by convection and radiation only.
It is also well known that conductive heat transfer is much more efficient than non-contact heat transfer at the typical temperatures utilized in processing plastic film. Further, it has also been found that uniform contact by a heater against a die is limited by surface roughness, rust and scale, clamping pressure, differential thermal expansion and other factors which are difficult to control.
As a result of the above factors there is large spot-to-spot variation of heat flowing into the surface of a plastic film extrusion die being heated with electrical resistance heaters which causes large temperature variations within such die; and, these variations cause difficulties in processing the film and result in poor quality film.
In particular, it is well known that when heat degradable polymers are exposed to an erratic and poorly controlled environment there is a greater tendency for such polymers to degrade. Further, differential thermal expansion within a die causes warpage thereof and creates undesirable internal stresses which distort the flow channels and thus cause poor gauge uniformity of the die and hence poor gauge uniformity of the plastic film being extruded from such die. Finally, variations of melt viscosity due to temperature differences cause gauge and profile control problems in the associated die.
Temperature variations that result from non-uniform surface heat rates are reflected as temperature variations at the melt wetted surface and cause degradation of the polymer and poor gauge control in the die. In general, there is some smoothing effect of melt wetted surface temperature in a die because heat spreads from the hottest spot in the die toward the coolest spot in such die; however, in the currently used dies this is not rapid enough.
It has been found that the temperature variations at the melt wetted surfaces could be substantially reduced in amplitude or smoothed, if not entirely eliminated, if the resistance to circumferential heat conduction could be significantly decreased and the resistance to radial conduction significantly increased. Thus, if the spreading of heat from hot spots to cold spots could be accelerated by using materials having better circumferential heat conductivity than radial heat conductivity there would be minimum temperature variations of a plastic melt flowing through a die.